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226                               ORDINARY BRASSES

= 1-1-5 amp. ;   voltage = 4-5 ;   duration,  30-40  minutes.    During the
whole course of the electrolysis, the liquid should be kept faintly acid.

4.  Determination of the Nickel.—In presence of nickel, the nickel
and zinc are determined under the conditions indicated for the electrolytic
analysis of argentan.

5.  Determination of the Phosphorus.—See Analysis of Phosphor

6.  Determination of the Arsenic,  Sulphur  and  Bismuth.—The
methods given for the analysis of copper (q.v.) are followed.

B. Gravimetric Methods

In a small covered beaker, 0-5 gram of the sample is dissolved in 10-
15 c.c. of nitric acid (D 1-2) at a gentle heat.

1.  Determination of the Tin.—When the action is complete, the
solution is diluted with 15-20 c.c. of water and, if it appears turbid owing
to the presence of metastannic acid, evaporated to dryness.    The residue
is heated for some time with a few drops of nitric acid and a little water
and the metastannic acid filtered off, washed with hot water acidified with
nitric acid, ignited and weighed :   Sn02 X 07881 = Sn.

2.  Determination of the Lead.—The liquid freed from the metastannic
acid, or the original solution if the alloy does not contain tin, is treated
with 3-4 c.c. of concentrated sulphuric acid, evaporated to dryness and
heated on a sand-bath until white fumes of sulphuric acid appear.    When
cool, the residue is taken up in 50 c.c. of water, gently heated and stirred
to dissolve the basic copper and zinc sulphates, allowed to cool and mixed
with 15 c.c. of 95% alcohol.   After standing for 1-2 hours, the lead sulphate
separated is collected on a Gooch crucible, washed first with a mixture of
60 c.c. of water, 10 c.c. of alcohol and 0-5 c.c. of sulphuric acid and then
with alcohol until the filtrate is neutral; the crucible is then dried in an
air-oven in a roomy porcelain crucible :   PbS04 X 0-6831 = Pb.

3.  Determination of the Copper.—The filtrate from the lead sulphate
is evaporated until the alcohol is completely expelled, the residue being
heated to boiling with 100 c.c. of water and the copper precipitated with
hydrogen sulphide.   The liquid is decanted on to a filter and the precipitate
washed first with saturated hydrogen sulphide solution containing about
20 c.c. of 2N-sulphuric acid in 100 c.c. and afterwards  with saturated
hydrogen sulphide solution alone ; the precipitate is dried and the copper
determined as cuprous sulphide :   Cu2S X 0-7986 = Cu.

4.  Determination of the Iron.—The filtrate is freed from hydrogen
sulphide, concentrated to a small volume, treated with a few drops of
hydrogen peroxide to oxidise the iron, boiled and rendered alkaline with
dilute ammonia.   After a short rest on the water-bath, the precipitated
ferric hydroxide is filtered, washed, ignited and weighed :  Fe203 X 0-6994
= Fe.   If the amount of ferric hydroxide is considerable, the precipitation
should be repeated to get rid of traces of zinc.

5.  Determination of the Zinc.—The liquid free from iron is neutralisedlack spots difficult to remove.